Measurements are to be made of the ionized calcium concentration in squid axoplasm using the photoprotein aequorin. Experimental conditions will be manipulated such that Ca influx is caused to vary (decrease in Nao, or in Vm or in Cao) and the effect on calcium concentration noted. Similar measurements will be made of the unidirectional Ca influx using radioactive calcium. The purpose of this study is to determine the effect of membrane potential on the mechanism responsible for the movement of calcium in both directions across the cell membrane.